1. Field of the Invention
The present invention generally relates to a method and apparatus for forming images and more particularly to a method and apparatus which is capable of applying a high fixing-nip pressure that is easily released when a recording sheet is stuck in a fixing mechanism.
2. Discussion of the Background
An image forming system that uses an electrophotographic method forms an electrostatic latent image on a photosensitive surface of an image support member (i.e., a photosensitive member) and develops the electrostatic latent image into a visible toner image using a developer including toner. The image forming system transfers the developed toner image onto a recording material such as a paper sheet, for example, and fixes the toner image to the recording material under heat and pressure with a fixing member.
The fixing member includes a pair of rollers serving as a pair of rotary members facing each other. As an alternative to the pair of rollers, a pair of belts or combination of a roller and a belt may be used. Also, as an alternative to the pair of rotary members, a pair of stationary members such as a pair of pressure pads can be used. As a further alternative, a combination of a rotary member and a stationary member can also be used.
One of the pair of rollers which is referred to as a heat roller includes a heater inside, and the other roller which is referred to as a pressure roller applies pressure to the heat roller. Thus, the pair of rollers which rotate together constantly form a pressure nip therebetween into which the recording material having the toner image is inserted and an image fixing process is performed as the recording material is moved forward by the pair of rollers.
When the image forming system causes a paper sheet jam in which the recording material is stuck in the fixing member, the system usually stops operations in order to allow operators to remove the recording material stuck in the fixing member. The recording material is, however, tightly held at the pressure nip by the pair of rollers and therefore it is not easy to remove the recording material.
To make the removal of jammed recording material easy, the image forming system is provided with a mechanism for releasing pressure, e.g., a pressure release lever.
There are increasing market demands for the background image forming system to become smaller and faster. On one hand, as a result of downsizing, the fixing member needs to be smaller and the width of the pressure nip for pressing and forwarding the recording material consequently becomes smaller. On the other hand, as a result of increasing a processing speed, the fixing member provides a smaller amount of heat to the recording material. Therefore, by these two phenomena, the fixing performance tends to be degraded.
To make the fixing member transfer a sufficient amount of heat to the recording material, the pressing force applied between the heat roller and the pressure roller needs to be increased so that the pressure nip becomes greater. If the pressing force is so increased, however, it becomes difficult to release the pressure between the heat roller and the pressure roller with the pressure releasing mechanism such as the pressure release lever.